Our recent studies indicate that the DNA binding factor Ikaros is required for pre-B cell transition from a proliferating to a differentiating phase. Upon Ikaros loss there is a dramatic accumulation of large proliferating pre-B cells and a block at this developmental stage. Proliferating pre-B cell are considered the normal developmental counterpart of B cell acute lymphoblastic leukemia (B-ALL), and a block at this stage potentially tumorigenic. IKAROS is frequently inactivated in human B-ALL, and its loss is observed in the majority of BCR-ABL B-ALL, defining a particularly aggressive and hard-to-treat leukemia. Here, we propose to establish the cellular and molecular pathways supported by Ikaros during pre-B cell differentiation and delineate the regulatory mechanisms involved. Ikaros is an integral component of a chromatin remodeling complex and functions by modulating chromatin in the vicinity of its target sites. Therefore part of this study aims at understanding the epigenetic regulation of the normal, pre- leukemic, and leukemic pre-B cell stages. In the first specific aim, we will delineate the effects of Ikaros loss on the cellular and molecular pathways that control transition through the pre-B cell stage. We will examine whether and how activity of Ikaros-dependent pre-B cell pathways are further modulated by BCR-ABL and establish a potential synergism or co-operation between the two factors in B-ALL. In the second aim, we will establish the gene networks that are directly regulated by Ikaros and the epigenetic mechanisms employed. Ikaros effects on chromatin accessibility at its gene target sites and on recruitment of other key transcriptional and chromatin regulators of this process will be evaluated. We will also evaluate whether the Ikaros-based regulatory process is also controlled by BCR-ABL. New genetic models based on Ikaros loss-of-function and gain of function for BCR-ABL will be combined with cutting-edge genome-wide gene expression, chromatin and signaling approaches to delineate the epigenetic, transcription and signaling networks effected by these factors during normal B cell differentiation and leukemia development. Exploitation of the Ikaros gene targets or their transcriptional and epigenetic mechanisms of regulation may empower the design of new intelligent/tailored diagnostics and therapies for high-risk B-ALL. .